The present invention relates to an apparatus for manufacturing a semiconductor device. More particularly, it relates to an apparatus for manufacturing a semiconductor device, wherein the apparatus employs a vacuum system.
In the fabrication of a semiconductor device using an apparatus employing a vacuum system, particles are formed during the pressure-reduction (pump down) of a process chamber and the particles formed thus settle on wafers being processed therein. Such particulate contamination of a semiconductor wafer significantly lowers the product yield and the reliability of a semiconductor device. This problem becomes more severe with increasing wafer diameters and with higher integration levels.
Particle formation during pressure-reduction or venting is attributed to a condensation mechanism caused by an adiabatic expansion of gas (see "Condensation-Induced Particle Formation During Vacuum Pump Down" by Yan Ye et al., Journal of the Electrochemical Society, Vol. 140, No. 5, pp. 1463-1468, May 1993). According to the suggested mechanism, a rapid drop in the pressure of a process chamber causes gas in the chamber to undergo adiabatic expansion and causes its temperature to fall rapidly. The gas is simultaneously condensed into water droplets. During droplet formation, such gases in the air as SO.sub.2, O.sub.3, H.sub.2 O and other gaseous impurities tend to diffuse into the droplets and thus become absorbed.
Due to the thermal capacity of the chamber walls being greater than that of the droplets, which are formed as described above, the inner surface temperature of the chamber decreases more slowly than does the temperature of the droplets. This resulting temperature difference causes heat to be transferred from the chamber to the droplets. Such heat transfer leads to a cycle of heating, evaporation and condensation of the droplets and causes an increase in the concentration of impurities in the liquid droplets. During the concentrated liquid phase, when the concentration of H.sub.2 O.sub.2 in the liquid is high enough, the formation of sulfuric acid droplets occurs. Namely, H.sub.2 O.sub.2 of the liquid quickly reacts with SO.sub.2 from the liquid or air to form sulfuric acid and, as the droplets continue their evaporation/condensation cycle, residue particles which are spherical in shape and contain mainly sulfuric acid are formed.
The main elemental components of the residue particles, which are formed by the evaporation/condensation cycles are carbon, sulfur and oxygen. The residue particles are quite stable thermodynamically and do not completely evaporate even upon heating to temperatures as high as 180.degree. C.
As the size of the process chamber is decreased the relative humidity of the gas increases and depressurizing speed or venting speed also increases. This leads to an increase in the number of condensation-induced particles that are formed.
FIG. 1 is a schematic view of an ion-implanting apparatus, in a conventional apparatus for manufacturing semiconductor devices, which employs a vacuum system. In FIG. 1, a reference numeral 10 denotes a process chamber for ion-implanting a semiconductor wafer therein; reference numeral 20 denotes two load lock chambers communicating with predetermined portions of the process chamber 10, for loading wafers to be transferred to the process chamber therein; reference numeral 30 denotes an isolation valve installed between the process chamber 10 and each of the load lock chambers 20, for providing isolation of the process chamber 10 from the load lock chambers 20; reference numeral 40 denotes a pump for reducing the pressure of the load lock chambers 20 to transfer the wafers to the process chamber 10 in a state of vacuum; reference numeral 50 denotes an exhaust pipe having two sub-pipes A connected to the respective load lock chambers 20 and a main pipe B of which one end is connected to the sub-pipes A and the other end is connected to the pump 40; and reference numeral 60 denotes two shut-off valves, one of which is installed in a predetermined portion in each of the sub-pipes A, for providing a shut-off of the flow of gas from the load lock chambers 20 to pump 40.
In the device of FIG. 1, an air valve, which is opened and closed depending on gas pressure, is usually used as the shut-off valve 60. The diameter of the main pipe B is larger than that of the sub-pipes A, since gas flowing from the load lock chambers 20 is collected in and passed through the main pipe B. Also, reference numeral 70 denotes an inlet for injecting venting-gas into the load lock chambers 20. The operation of a conventional apparatus for manufacturing a semiconductor device by employing a vacuum system will now be described in further detail while referring to FIG. 1.
The exhaust pipe 50 installed between the shut-off valves 60 and pump 40 is maintained in a low vacuum of about 10.sup.-3 Torr. Wafers are then loaded in the load lock chambers 20, and the shut-off valves 60 are opened to drop the pressure of the load lock chambers 20 to about 10.sup.-3 Torr. Thereafter, the wafers are transferred to the process chamber 10 by opening the isolation valves 30. Such process chamber 10 is maintained at a high vacuum of about 10.sup.-6 Torr during the processing of wafers.
The load lock chambers 20 are set to the lower vacuum to avoid a sudden turbulence of gas and to reduce the preliminary formation of particles before the wafers are transferred to the process chamber 10. Such formation of particles is minimized by reducing the difference between the high vacuum of the process chamber 10 and atmospheric pressure of the load lock chambers 20.
FIG. 2 is a sectional magnified view of the air valve which is used as a shut-off valve 60. In FIG. 2, a reference numeral 100 denotes a cylindrical case having a first orifice on its side and a second orifice on its base; reference numeral 200 denotes a board installed in a predetermined location inside of the case 100 to divide it into an upper portion and a lower portion and having a hole in the center thereof for communicating the lower portion of the case 100 with the upper portion of the case 100; reference numeral 300 denotes a bellows fixed to the board 200 and surrounding the center hole thereof which is sealed by being attachably fixed to the lower surface of the board and having an O-ring on the lower end portion thereof, to provide a means for opening and closing the second orifice at the base of the case 100; reference numeral 500 denotes an elastic rubber boot installed on the internal upper surface of the case 100 and extending in a convex manner in the direction of the lower portion of the case 100; reference numeral 600 denotes a spring support fixed to the convex surface of the rubber boot 500, one portion thereof being inserted into the hole of the board 200; reference numeral 700 denotes a spring, one end of which is fixed to a base 400 of the bellows 300 and the other end of which is fixed to the spring support 600; reference numeral 800 denotes an air inlet communicating with a side portion of the upper portion of the case 100; and reference numeral 900 denotes the flow of gas from the load lock chambers 20 via the first orifice located on the side of the case 100 to the pump 40 (FIG. 1) via the second orifice located in the base of the case 100.
If the pressure of air injected through the air inlet 800 is below a predetermined value, the second orifice located at the base of the case 100 is opened by the base 400 of the bellows 300. If the air pressure is greater than or equal to the predetermined pressure, the convex end portion of the rubber boot 500 is pushed upward, vertically contracting the bellows 300 and thus closing the second orifice at the base of the case 100. Since the second orifice is quickly opened, the pressure of the load lock chambers 20 also rapidly drops.
FIGS. 3A and 3B show scanning electron microspectroscopy (SEM) photographs of particles formed in the load lock chambers 20. The particles are about 0.3 .mu.m to 3.7 .mu.m in diameter and generally spherical in shape.
FIG. 4 illustrates the result obtained by analyzing the components of the particles of FIGS. 3A and 3B by Auger electron spectroscopy (AES). The main components, as shown by the elemental analysis graph of FIG. 4, are sulfur, oxygen and carbon.
As described above, in the conventional apparatus for manufacturing a semiconductor device by employing a vacuum system, gas condensation is found to be the cause of particle formation. This conclusion is supported by an analysis of the shape and components of particles generated in the load lock chambers, as suggested by Yan Ye et al. In other words, as the shut-off valves installed on the exhaust pipe for exhausting the load lock chambers of gas are rapidly opened, the pressure of the load lock chambers drops quickly. Therefore, particle formation is attributed to adiabatic expansion of the gas in the load lock chambers.
By the same mechanism as described above, the adiabatic expansion of venting-gas also produces particles during the venting of the load lock chambers.